1. Field of the Invention
The present invention relates to the known field of offshore drilling from an anchored floating support on the surface, and more particularly it relates to seabed devices for guiding drill strings.
The invention relates more particularly to deflected drilling in deep water in order to reach points that are remote from vertically below the drilling equipment on the surface.
2. Discussion of Related Art
Once the depth of water becomes great, production fields and in particular oil fields are generally explored and operated from a floating support. As a general rule, the floating support has anchor means for keeping it in position in spite of the effects of current, winds, and the swell.
For drilling operations, it generally also has means for handling drill strings, such as guide equipment and associated safety systems installed on the seabed.
Wells are normally drilled vertically below the drilling equipment, and then they penetrate vertically into the ground over depths of several hundreds of meters. Thereafter, such wells are continued towards the oil deposit, referred to as a “reservoir”, either in a vertical direction or else with the well being progressively deflected angularly so as to reach points of said reservoir that are remote to a greater or lesser extent.
By drilling a plurality of deflected wells in this way, it is possible to build up an array of wells in an umbrella shape extending from a common position for the floating support on the surface, thus making it possible to bring together all of the surface equipment in a single location throughout the time the field is being worked. Such installations are referred to dry tree units (DTU), i.e. well head units that are said to be “dry” because the well heads are brought together at the surface out of the water. This makes operation much easier since it is possible to have access to any of the wells from the DTU in order to perform any control or maintenance operations on a well, and this continues to be true throughout the lifetime of an installation which may be as much as 20 years to 25 years, or even longer.
It is possible to drill such wells only if the reservoir is at great depth, e.g. 2000 meters (m) to 2500 m, since it is essential to have a vertical length of several hundreds of meters in the seabed prior to initiating deflection of the well with the radii of curvature of the pipework constituting a well being of the order of 500 m to 1000 m.
Patents EP 0 952 300 and EP 0 952 301 disclose methods and apparatuses enabling deflected wells to be bored while taking advantage of the depth of water to go as far as possible away from vertically beneath the drilling equipment and in order to rest on the seabed in a manner that is substantially tangential to the horizontal.
In those patents, the guide devices installed on the seabed penetrate into the ground and enable a borehole to start being drilled into the seabed at an inclination of given angle relative to the vertical. The guide device is connected to the drilling equipment via a pipe referred to as a “drilling riser” which guides the drill string that passes through it and which serves to raise the drilling mud and debris.
The guide element installed on the seabed must ensure that large radii of curvature of 500 m to 1000 m are complied with and consequently it must be of large dimensions, while nevertheless remaining very strong in order to be able to absorb the considerable forces that are generated by the drilling string which is also constrained to follow the same radius of curvature, thereby giving rise to very high levels of friction and to a risk of the assembly becoming destabilized during drilling.
In addition, the guide element of considerable dimensions and mass must be preinstalled in ultradeep water, i.e. in water having a depth of 1000 m to 2500 m, or even more.
More precisely, EP 0 952 301 discloses a guide device which comprises a pipe element referred to as a “conductor” which is the guide tube of the borehole deployed from the floating support through the drilling riser down to a structure referred to as a “skid” resting on the seabed. This skid structure holds and guides the conductor tube horizontally at a certain height above the seabed. Thereafter the conductor adopts a curve towards the seabed under the effect of its own weight. While it is being deployed, the conductor co-operates with drilling tools so as to become embedded in part in the seabed. Putting such a guide device into place and in particular putting the conductor into place from the floating support represents a major operating constraint. In addition, the guide device does not provide for any control over the curvature of the conductor. Furthermore, in order to comply with a large radius of curvature, in particular a radius greater than 500 m, it is necessary for the conductor to be deployed tangentially to the horizontal over several tens of meters beyond the support point which guides it on the skid structure.
Finally, no means are described in those patents for enabling said conductor to be put into place with a large radius of curvature as is necessary to ensure that the drill string, and above all the casing elements, can operate with a minimum amount of lateral friction within the pipe.
For a radius of 600 m, if the well head is at 2 m above the sea bottom, the conductor will reach the ground only 50 m further away, which means that a 50 m length of conductor is cantilevered out freely and unsupported, which is unacceptable since the conductor runs the risk of breaking or kinking with curvature that is too sharp since it is not controlled. Furthermore, the cantilever created in this way can be harmful to proper operation during drilling operations and indeed throughout the lifetime of the well which may exceed 25 years.